"Cancer may afflict him in many ways, but it will not make him surrender," she wrote. "Nothing ever has."

Gupta said McCain went to his doctor's office Friday morning "basically for a scheduled annual physical exam."

"He was complaining a bit of fatigue and said he's been feeling tired over the last few months," Gupta said. "He also had a bout of double vision. But because of those things his doctors decided to order a cat scan of his brain."

CBS News reported the family and Mayo Clinic released a statement about the grim diagnosis.

"On Friday, July 14, Sen. John McCain underwent a procedure to remove a blood clot from above his left eye at Mayo Clinic Hospital in Phoenix," the statement read. "Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot."

The statement also added the 80-year-old senator and his family are reviewing further treatment options.

"Treatment options may include a combination of chemotherapy and radiation," it added. "The senator's doctors say he is recovering from his surgery 'amazingly well,' and his underlying health is excellent."

McCain's office released a statement in wake of the recent revelation.

"Sen. McCain appreciates the outpouring of support he has received over the last few days," the statement read. "He is in good spirits as he continues to recover at home with his family in Arizona. He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective. Further consultations with Sen. McCain's Mayo Clinic care team will indicate when he will return to the United States Senate."

A CAT scan and subsequent MRI scan revealed the "abnormality" and "within the next couple of hours, the doctors took him to the operating room, they performed the incision in the left eyebrow area" and removed the tumor from the area of the left front of his brain, Gupta reported.

"It's the same type of tumor that Sen. Kennedy had, that Bo Biden had," Gupta said. "It is an aggressive type of brain cancer. So Sen. McCain and his family are dealing with the news and trying to decide the next steps in terms of treatment. "

He is currently at home, Gupta reported, and had a "rapid recovery" from the surgery.

The potential next steps, he said, might include chemotherapy and radiation, but "there's not a type of treatment you can say is going to likely lead to cure."

"He has never shied from a fight, and I know that he will face this challenge with the same extraordinary courage that has characterized his life," McConnell said in the statement, adding: "We all look forward to seeing this American hero again soon."

President Donald Trump issued a statement sending "thoughts and prayers" to McCain — and noting he's "always been a fighter."